The present invention relates to a polygonal throw away insert which is detachably mounted on a tool holder.
Various kinds of throw away inserts having chip breaking means are commercially available, but their chip breaking capability and working strength are not necessarily satisfactory.
FIGS. 1 to 4 show a prior art triangular throw away tip or insert for both right-hand and left-hand use. This insert is provided with a chip breaking groove 52 of uniform width along a cutting edge 50 and a land 51 throughout its periphery. Some difficulties which are experienced in cutting with this type of throw away insert will be described below. Assuming that the width a of groove 52 is 3 mm, and the radius r of a nose 53 is 1.2 mm and the side cutting edge angle is 0.degree. (FIG. 11), when the depth of cut b is not less than 4.31 mm (a cot 30.degree.-r cot 30.degree.+r=4.31), the groove 52 serves satisfactorily as a chip breaker. However, when the depth is less than 4.31 mm (e.g. 3.0 mm as shown at c), the apparent breaking width d ({3.0 mm+(r cot 30.degree.-r)} tan 60.degree.=6.72 mm) will be too wide for effective chip breaking. This will be understood from FIGS. 5 and 6 in which letter A designates the workpiece, B the insert, and C the chip produced in cutting. By narrowing the width of the breaking groove, the insert can be applied to cutting of a smaller cutting depth. However, this would cause choking or clogging with chips at a large depth of cut.
FIGS. 7 to 9 show another prior art triangular insert of this kind. The insert has a cutting edge 50, a land 51 and a ramp 54 sloping gently toward its center, and is provided with a semicircular projection 55 adjacent each nose 53 so that its center positions on the bisector of the nose, and a pair of small projections 56 of similar shape behind the projection 55 along the respective cutting edge. These projections 55 and 56 serve as a chip breaker. This type of throw away insert also results in difficulties similar to those mentioned above. It cannot control the chips satisfactorily when the depth of cut is small.
In case of double-sided inserts, if an appropriate amount of breaker width is given, the amount of overhang, k or l, (FIGS. 1 and 7) at the nose would be considerably large. This means insufficient support strength.
Furthermore, if such prior art throw away inserts were used as a double-sided insert, they would be supported on the cutting edges 50 and the projections 55, 56, namely on lines and points only. Clearly, this manner of support could not provide sufficient strength.
Also, such prior art throw away inserts are capable of controlling chips in internal or external cuttings by use of their nose, in which the chips curl up relatively easily into an easy-to-break shape because the thickness of chip is not uniform. In contrast, in cutting with a central portion of a cutting edge, e.g. in chamfering, the chip produced is more difficult to break because its thickness is uniform.